In surgeries involving spinal fixation, interbody cages are often used to restore disc space height, serve as a conduit for bone graft, and to help immobilize vertebrae undergoing fusion. Distracting the disc space prior to cage insertion restore disc space height. Distraction serves two important functions. First, it can decrease pressure on spinal nerves by increasing the size of the intervertebral foramen. Second, distraction increases tension on the annulus fibrosis which, in turn, increases the stability of the vertebra-cage-vertebra construct.
Presumably the annular tension decreases with time, thus weakening the construct. Furthermore, the annulus is weakened in many patients with severe degenerative disc disease. Given these and other deficiencies with annular tension, additional fixation is frequently added to increase the rigidity of the vertebra-cage combination.
Currently such additional fixation is inserted onto or into the posterior aspect of the spine. Thus, patients who have cages inserted from an anterior approach must undergo a second operation from the posterior aspect of the body. As might be expected, the second surgery increases patient morbidity, insurance costs, and delays return to work.
There are two ways to insert supplemental fixation through the same incision. One technique uses the interbody cages disclosed in my co-pending U.S. patent application Ser. No. 09/454,908, the entire contents of which are incorporated herein by reference. Posterior insertion allows the addition of supplemental fixation through the same incision.
A second solution employs fixation inserted through the anterior aspect of the spine. With known anterior lumbar spine fixation techniques, a combination of screws and rods or plates are inserted on the lateral side of the vertebrae from an anterior or lateral approach. The fixation is placed on the lateral aspect of the spine to avoid the aorta. Previous metal devices placed under the aorta have lead to aneurysms in some cases (Dunn Device). Unfortunately, a few patients have died from rupture of the aneurysms.
Lateral fixation is not ideal with interbody cages. First, lateral fixation cannot be used at the L5-S1 level. The iliac arteries cross the L5-S1 level anteriorly and laterally. Second, the vascular anatomy of many patients does not permit lateral fixation at the L4-L5 level. The majority of cages are inserted at the L4-L5 and L5-S1 levels. Third, cages are generally inserted in a directly anterior-to-posterior fashion with the patient in a supine position. Lateral instrumentation is difficult if not impossible in most patients in the supine position.
The system described in U.S. Pat. No. 5,904,682 uses two flat plates applied to screws placed bilaterally on either side of the disc space. The system does not use cables or diagonal bracing to resist rotational forces. In U.S. Pat. No. 4,854,304 screws laced in the side of the vertebral bodies are connected from a lateral approach. The screws are connected with a threaded rod. In 1964, A. F. Dwyer described a system using a single cable to connect screws placed on the lateral portion of the vertebral bodies. Dr. Dwyer connected a series of screws with one screw per vertebral body. The arrangement described in U.S. Pat. No. 4,854,304 is similar to Dr. Dwyer's system, but the cable is replaced with a threaded rod. Dr. Ziekle modified Dr. Dwyer's system in 1975, as set forth in U.S. Pat. No. 4,854,304.
Cables and tensioning devices are also well known in orthopedic spine surgery. References that use cables include U.S. Pat. Nos. 4,966,600; 5,423,820; 5,611,801; 5,702,399; 5,964,769; 5,997,542. None use diagonal members to enhance compression and resist lateral movement, however.